Broadband absorbers via nanostructures

ABSTRACT

The document discloses transferrable hyperbolic metamaterial particles (THMMP) that display broadband, selective, omnidirectional absorption and can be transferred to secondary substrates, allowing enhanced flexibility and selective transmission. A device having metamaterial nanostructures includes a substrate and metamaterial nanostructures engaged to the substrate to form an optical layer to interact with light incident to the optical layer to exhibit optical reflection or absorption or transmission that is substantially uniform over a spectral range of different optical wavelengths associated with materials and structural features of the metamaterial nanostructures, each metamaterial nanostructure including different material layers that are interleaved to form a multi-layer nanostructure.

PRIORITY CLAIM AND CROSS REFERENCE TO RELATED APPLICATIONS

This patent document is a divisional of U.S. patent application Ser. No.16/479,892, fil. on Jul. 22, 2019, which is a 371 National PhaseApplication of PCT Application No. PCT/US2018/014919, filed on Jan. 23,2018, which claims the benefit of priority of U.S. Provisional PatentApplication No. 62/449,507, filed on Jan. 23, 2017. The entire contentsof the before-mentioned patent applications are incorporated byreference as part of the disclosure of this document.

TECHNICAL FIELD

This patent document relates to systems, devices, and processes relatedto materials based on nanoparticles or nanostructures.

BACKGROUND

Various natural or man-made materials exhibit material properties basedon their material compositions. With advancement in materialengineering, various artificially engineered structures based oncombinations of different materials and structures have been or arebeing developed as “metamaterials” that exhibit new and useful materialproperties or behaviors. Notably, various metamaterials may be designedto exhibit unique properties, which are not attainable with naturallyoccurring materials and arise from their specially designed “meta-atom”structure rather than their chemical compositions. For example, somemetamaterials have been shown to have a negative permittivity ϵ, anegative permeability μ, or a negative refractive index n (i.e.,negative ϵ and μ simultaneously) at the desired electromagnetic wavespectral ranges.

SUMMARY

The present document discloses material structures formed bynanostructures to exhibit plasmonic responses that can be engineered forvarious applications. For example, the disclosed material structures canbe designed to absorb light over a broadband spectral range, includingsome spectral ranges that are used to achieve desired optical propertiessuch as infrared or far-infrared ranges, to have a nearly constantoptical absorption over wide incident angles or over different opticalpolarizations. The disclosed material structures can be fabricated byfirst forming their constitutional nanostructures over a substrate andthe formed nanostructures or layers of such nanostructures can betransferred onto other substrates and/or self-assembled onto substrates.

In one example aspect, a device having metamaterial nanostructures mayinclude a substrate; and metamaterial nanostructures engaged to thesubstrate to form an optical layer to interact with light incident tothe optical layer to exhibit optical absorption or transmission that issubstantially uniform over a spectral range of different opticalwavelengths associated with materials and structural features of themetamaterial nanostructures, each metamaterial nanostructure includingdifferent material layers that are interleaved to form a multi-layernanostructure.

In another example aspect, a device having metamaterial nanostructures,comprising a substrate that is flexible; and an array of metamaterialnanopillars engaged to the substrate to form an optical layer, eachmetamaterial nanopillar including alternating semiconductor anddielectric layers to exhibit a plasmonic response to incident light toexhibit optical absorption or transmission that is substantially uniformover a spectral range of different optical wavelengths associated withmaterials and structural features of the metamaterial nanopillars.

In another example aspect, a method for making a device havingmetamaterial nanostructures, comprising forming metamaterialnanostructures on a first substrate by depositing and patterning thedeposited materials on the first substrate to form distinctivestructures on the first substrate as the metamaterial nanostructures,wherein materials and structural features of the metamaterialnanostructures are designed to exhibit optical absorption ortransmission that is substantially uniform over a spectral range ofdifferent optical wavelengths, separating the metamaterialnanostructures from the first substrate, and transferring themetamaterial nanostructures onto a second substrate that is differentfrom the first substrate.

In another example aspect, a layered metamaterial structure comprising afirst layer comprising a dielectric material and a second layercomprising a plasmonic material is disclosed. The plasmonic material hasa carrier concentration that exhibits plasmons in a target operatingfrequency band. The structure comprises one or more sphericalnanoparticles in which a number of concentric first and second layersalternate with each other. The structure comprises one or morecylindrical nanotubes comprising coaxial layering of the first layer andthe second layer along a longitudinal axis. The structure furthercomprises an air core at the longitudinal axis.

In another example aspect, a structure with multilayered metamaterialsmay include alternating layers of dielectric and plasmonic materials,and a secondary substrate to which the alternating layers of thedielectric and plasmonic materials formed on a primary substrate istransferred. The plasmonic material has a carrier concentration thatexhibits plasmons in a target operating frequency band. The alternatinglayers of dielectric and plasmonic materials have a sphere shape inwhich a number of concentric dielectric and plasmonic layers alternatewith each other. The alternating layers of dielectric and plasmonicmaterials have a cylinder shape in which a number of coaxial dielectricand plasmonic layers alternate with each other.

In another example aspect, a method of fabricating an array ofnanoparticle structures. The method includes generating a silicon holearray template using nanosphere lithography, filling in holes of thesilicon hole array using atomic layer deposition by multilayeredstructures of hyperbolic metamaterials (HMMs), etching, using a reactiveion etching or wet etching process, a top layer of result of the fillingoperation, thereby exposing underlying silicon of the silicon holearray, and removing silicon surrounding hyperbolic metamaterial to leavebehind an array of packed multilayered structures of HMMs.

In applications, various structures based on the disclosed technologycan be used to absorb light with optically thin materials for manyapplications such as camouflage, light detection and energy harvestingand may be transferred to more desirable substrates for mechanicallyflexible and low-cost applications. The disclosed material structurescan be arranged as arrays or close-packed configurations to showbroadband, selective, omni-directional, optical absorption.

The above aspects, their implementations and applications are describedin detail in the drawings, the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A)-1(E) illustrate schematic geometries of (a) multilayer and(b) nanowire based hyperbolic metamaterials (HMMs), (c) sphericalisofrequency surface for an isotropic dielectric, and hyperboloidisofrequency surface for a uniaxial medium with (d) one negativecomponents and (e) two negative components of the dielectric tensor.

FIGS. 2(A) and 2(B) illustrate two examples of hyperbolic metamaterialparticles in the form of (A) hyperbolic spheres and (B) hyperbolictubes.

FIG. 3 is an example schematic of coupled hyperbolic nanotubes (HNTs).

FIGS. 4(A) and 4(B) show simulated absorption of (A) the exact structureand an effective medium approximation (EMA) to describe the HNT shelland (B) EMA HNT arrays with air cores and solid hyperbolic metamaterialnanowires.

FIG. 5(A) shows examples of simulated absorption of HNTs with increasingn from 1-8 and 13 with a constant inter-pillar gap distance. FIG. 5(B)shows simulated absorption of exact structure HNT arrays with varying aand constant Dt. FIG. 5(C) shows simulated absorption of EMA solidhyperbolic nanowire arrays of varying diameter. FIGS. 5(D) and 5(E) showsimulated absorption using the exact structure of HNTs with varying (D)lengths and (E) filling factor of metallic material ff_(m) in the shell.FIG. 5(F) shows simulated absorption using the exact structure of HMIscompared with the HNTs.

FIG. 6 shows effective material dispersion.

FIG. 7 compares a photonic hypercrystal (PhHC) of constant dH=740 nm and‘a’ increasing from 740 nm to 3500 nm.

FIGS. 8(A) and 8(B) show scanning electron microscope (SEM) images ofthe HNT arrays as viewed in the plane (a) parallel and (b) perpendicularto the nanotube axis.

FIG. 9(A) shows absorption spectra of HNT arrays deposited at atemperature of 185° C. and 200° C. along with a spectrum of a purealuminum-doped zinc oxide (AZO) nanotube array deposited at 200° C. FIG.9(B) illustrates schematic of the incident radiation at angle 0 showingtransverse-electric (TM) polarization. FIGS. 9(C) and 9(D) showwide-angle absorption spectra for (C) TM and (D) transverse-magnetic(TE) polarizations of an HNT array deposited at 200° C.

FIG. 10(A) shows comparison of experimental and simulated absorptionspectra for sample A. FIG. 10(B) shows transmission, and FIG. 10(C)shows absorption spectra for samples A-C with parameters as described inTable 1 after flexing several times as shown in FIG. 10(E). FIGS. 10(D)and 10(E) show photographs of sample A showing (D) visible transparencyand (E) mechanical flexibility. FIG. 10(F) shows SEM image of an HNTarray (Sample B) after transferring to a flexible polydimethylsiloxane(PDMS) substrate and flexing several times as shown in FIG. 10(E).

FIG. 11 illustrates an example infrared (IR) reflection spectrum of anHNT array.

FIGS. 12(A) and 12(B) depict an example of a simulated absorptionspectra of a closely packed TiN/(Al,Sc)N based (A) HNT array and (B)hyperbolic nanosphere array. The HNT array parameters L, Dt, Dc, g, a,n, tm and td used in the simulation were 750 nm, 500 nm, 60 nm, 30 nm,530 nm, 11, 10 nm, and 10 nm, respectively. The Dt, Dc, g, a, n, tm, andtd used for the hyperbolic nanosphere simulation were 540 nm, 60 nm, 30nm, 570 nm, 12, 10 nm and 10 nm, respectively.

FIGS. 13(A) to 13(I) are cross-section diagrams that show an example HNTfabrication process.

FIGS. 14(A)-14(D) illustrate (a) scanning electron micrograph of a Sinanopillar array coated with a 56 nm AZO layer deposited via ALD, (b)cross-sectional image of a single Si nanopillar with an AZO coating, (c)IR extinction spectra of AZO-coated Si nanopillars at a constantdeposition temperature (200° C.) but varying Al/Zn compositions, (d)log-log plot of carrier concentration vs. resonant wavelength forvarious AZO-coated Si nanopillar arrays, where the dashed line is thelinear relationship expected from a Drude model.

FIGS. 15(A)-15(D) illustrate (a) real and (b) imaginary dielectricconstants of AZO and thermally processed AZO (825° C.) usingellipsometry. Samples are coated with a 12 nm thick HfO2 passivationlayer, (c) IR extinction spectra of thermally processed AZO/HfO2 (56/12nm thicknesses) after depositing on a Si nanopillar array and performingRTP treatment at different temperatures, (d) comparison of experimentaland finite-difference time-domain (FDTD) simulation of an AZO/HfO2(56/12 nm thicknesses) layer coated on a Si nanopillar array.

FIG. 16 illustrates jumping particle ALD based on proposed fabricationscheme for 3D multilayer HMM nanospheres.

DETAILED DESCRIPTION

Coating a structure with a material that has different optical,radiative or absorptive characteristics can make the structure look tobe something different when viewed, scanned or sensed from distance.Such altering of properties of the structure, or the substrate, can havemany useful industrial applications in areas such as telecommunications,and military applications such as camouflaging. In the past, scientistshave attempted to design the applicative material. However, for thereasons discussed herein, such material has found limited use inpractical application due to several reasons.

One such material that has been studied by researchers is carbonnanotubes. Carbon nanotubes show high, broadband and omnidirectionalabsorption. However, films made from carbon nanotubes are relativelythick. In addition, it is impossible to select the spectral region inwhich absorption occurs. Hyperbolic metamaterial (HMM) pyramids showbroadband absorption. However, they have only been proven by low meltingpoint noble metals, and omnidirectional absorption has not beenexperimentally determined and is only expected for transverse magnetic(TM) polarizations. Further, many designs use metallic substrates.Perfect metamaterial absorbers coupled to material losses have also beenshown. These materials also rely on planar back reflectors anddielectric spacers. This is problematic because planar metallicmaterials cannot be flexed, are generally not optically transparent, andsuch materials possess a wide surface area, which makes thermal latticemismatch an issue for high temperature applications.

The field of plasmonics has grown at an aggressive pace over the pasttwo decades, fueled mainly by a host of exciting applications whichincludes new optical interconnects, cancer therapeutics, biosensors,spectroscopy, hyperbolic dispersion, light emission, and guidance oflight in subwavelength features. Most plasmonic materials consist ofnoble metals such as gold or silver due to their strong resonances inthe visible part of the electromagnetic spectrum, but this poses animmediate problem for complementary metal-oxide-semiconductor (CMOS)technologies since pure metal components are not well-suited for CMOSprocessing and they are expensive. Furthermore, the plasmonic responsesare fixed in the visible which makes plasmonic responses in the infrared(IR) limited. Active plasmonic materials in the IR (particularly around1550 nm) could play a significant role in areas such as energyfiltering, broadband absorption, telecommunication optical modulations,and IR light generation. Therefore, alternative materials such ashighly-doped semiconductors have been gaining significant traction dueto their tunable plasmonic properties in the IR. However, it is stilldifficult to consistently synthesize plasmonic materials that havelow-loss in the IR region and even more challenging to fabricatematerials that can be easily transferred to secondary substrates ormechanically flexible surfaces without sacrificing performance. It istherefore timely to make a strong push towards improving the opticalresponse of highly-doped semiconductors and utilizing their propertiesto engineer novel plasmonic materials for high-performance devices.

The disclosed technology in this patent document apply nanoengineeringtechniques to develop a new class of transferrable hyper-crystals (THC),constructed of an array of hyperbolic metamaterial (HMM) units thatexhibit strong optical anisotropy and can be easily transferred tosecondary substrates while maintaining their optical performance.Incorporating HMMs into the unit cell of a material provides a newdegree of freedom in designing materials with unique properties, whilethe transferable characteristics remarkably enhance their practicality.In implementing the disclosed technology, various geometries or shapesof hyperbolic metamaterial (HMM) units may be used, such as nanotubes,nanocylinders, nanospheres and others. The experimental realization ofthe proposed THC relies on developing low-loss, IR tunable plasmonicsemiconductors that can be conformally deposited on nanoscale featuresusing techniques such as atomic layer deposition (ALD). As aconcept-proof-experiment, the THC will be used to demonstrate selective,omnidirectional broadband absorbers.

When light interacts with a highly polarizable material (i.e., containsfree electrons) the electrons collectively oscillate in response to theelectromagnetic wave which creates a plasmon. When the plasmon isconfined to a surface such as in the case of a nanoparticle it is termeda localized surface plasmon resonance (LSPR). These LSPRs have been atthe core of many of the plasmonic applications since they are highlytunable, dependent on the chemical composition of the material,surrounding medium, shape, and size, and can significantly amplify thelocal fields near the surface. One of the most important parameters forLSPRs is the oscillator strength, f, which is proportional to the numberof free electrons in the material. The oscillator strength of plasmonicmaterials can reach ˜100, compared to ˜1 for fluorescent nanomaterialssuch as a quantum dot, which explains why there are such stronglight-matter interactions with plasmonic materials (scatteringcross-section is proportional to f²).

One of the other key fundamentals in plasmonic physics, other than theoscillator strength, is the modal volume, V_(m), which is the effectivevolume that the LSPR takes up. This is important since it isinter-related to the oscillator strength and losses of the plasmonicresonances. In general, V_(m) is on the order of 10 nm³ for nanomaterialresonances in the visible part of the spectrum, but if the size of thematerial is larger than the skin depth (i.e., penetration depth of theEM wave before decaying to 1/e of its original value) than the LSPRs canbecome extremely lossy for traditional noble metals such as gold andsilver. These metals are the preferred plasmonic materials in thevisible regime due to their small ohmic losses or DC conductivity, butother losses begin to dominate at other wavelengths namely thehigher-frequency interband losses (e.g., promotion of valence electronsto the Fermi surface) and lower-frequency intraband (e.g., free electronlosses) or Drude losses. A closer look at the metal's free-electronresponse to light describes these losses in terms of the dielectricpermittivity, ϵ, and magnetic permeability, μ. Since the magneticinteraction is weak for optical frequencies, μ is close to one andpermittivity remains the main parameter to understand the light-matterinteractions.

The material permittivity is a complex quantity, requiring both a real(ϵ′) and imaginary (ϵ″) part to properly describe the dielectricfunction. The real part is related to the polarization response of themetal and the imaginary part is related to the losses (i.e., interband,intraband, and additional scattering losses). Metals operating atresonant frequencies have a negative real permittivity and a non-zeroimaginary permittivity. The Drude model helps describe the behavior ofplasmonic materials by treating the electrons as a free-electron gas or15 plasma. The dielectric constant of a plasma is given as ϵ(ω)=1−ω_(p)²/ω² where ω_(p) is the bulk plasma frequency defined as

$\omega_{p} = {\sqrt{\frac{ne^{2}}{\varepsilon_{om_{e}}}}.}$

In the ω_(p) expression n is the majority carrier concentration, e isthe elemental charge, ϵ₀ is the free space permittivity, and m_(e) isthe effective mass of the electron. Using a damping coefficient (γ) anda background permittivity (ϵ_(∞)), to take into account electroncollisions and the dielectric response of bound atomic charges,respectively, a complex permittivity expression can be written suchthat:

$\begin{matrix}{{\varepsilon(\omega)} = {{\varepsilon^{\prime} + {i\varepsilon^{''}}} = {\varepsilon_{b} - \frac{\omega_{p}^{2}}{\left( {\omega^{2} + \gamma^{2}} \right)} + {i\frac{\omega_{p}^{2}\gamma}{\left( {\omega^{2} + \gamma^{2}} \right)\omega}}}}} & {{Eq}.(1)}\end{matrix}$

The damping coefficient, or relaxation rate, is directly linked to thescattering/ohmic losses and is directly proportional to the imaginarypart of the dielectric function. Likewise, ω_(p) ² is proportional tothe carrier concentration (n) and also scales directly with ϵ″.Therefore, to 25 decrease losses in a plasmonic material, either therelaxation rate, carrier concentration, or both, have to be reduced.Conventional noble metal plasmonics have extremely large n (˜10²³ cm⁻³)which causes ϵ″ to be very large in the IR and limits their use at thesefrequencies. In addition, the large polarizability of the noble metalsprohibits their use in external field driven applications such aselectrochromics, optical modulations, and transistors. One approach topush LSPRs out into the IR and allow low loss plasmonic systems (ϵ″≈0)is to use materials such as heavily-doped semiconductors.

Creating semiconductors that behave like metals (ϵ′<0) at NIRwavelengths (e.g., λ=1.55 μm) is achieved by doping to levels>10²⁰ cm⁻³.However, this poses major technical difficulties and serious constraintson the choice of the material. Furthermore, at high dopingconcentrations the mobility (μ) of the carriers decreases which causesthe relaxation rate (and hence ϵ″) to increase according to γ=e/μm_(e)where m_(e) is the effective mass of the carrier. Reducing the lossesrequires that the product of the effective mass and mobility be as largeas possible which puts upper limits on the dopant concentration. Anotherissue that needs to be dealt with when choosing a semiconductor is theoptical band gap. If the gap is similar or smaller than the plasmonfrequency this will induce further optical losses due to interbandtransitions. Taking all these parameters into consideration leavesseveral prime semiconductor candidates for plasmonic materials includingsilicon, germanium, III-V compound semiconductors (e.g., GaAs, InP,GaN), and transparent metal oxides (e.g., zinc oxide [ZnO], indium tinoxide). Of these materials, aluminum-doped zinc oxide (AZO) has become aleader for plasmonic applications given its large optical band gap andability to achieve high doping levels with lower loss than any othermaterial reported. Therefore, AZO will be the main material platforminvestigated in the proposal to fabricate the NIR broadband absorbingmaterials and devices, but other materials such as nitrides (e.g., TiN)will also be explored because of their pronounced responses in thevisible.

FIGS. 1(A)-1(E) illustrate schematic geometries of (a) multilayer and(b) nanowire based HMMs, (c) spherical isofrequency surface for anisotropic dielectric, and hyperboloid isofrequency surface for auniaxial medium with (d) one negative components and (e) two negativecomponents of the dielectric tensor. The HMMs can support waves withextremely high k-vectors. HMMs are one type of anisotropic materialsystems that possess permittivities with opposite signs along orthogonaldirections. Since metals and dielectrics have negative and positivepermittivities, respectively, at optical frequencies, HMMs may be madeof metal-dielectric composites. When the features are much smaller thanthe working wavelength, the HMMs can be described by an effective mediumapproximation (EMA). The effective permittivities ϵ_(x), ϵ_(y) and ϵ_(z)in the x, y and z directions can be tuned by the metal filling factor p,and calculated by the following equations in the 1D multilayer (Eq. 2)and 2D nanowire systems (Eq. 3):

$\begin{matrix}\left\{ \begin{matrix}{\varepsilon_{x} = {\varepsilon_{y} = {{p\varepsilon_{m}} + {\left( {1 - p} \right)\varepsilon_{d}}}}} \\{\varepsilon_{z} = \left( {\frac{p}{\varepsilon_{m}} + \frac{1 - p}{\varepsilon_{d}}} \right)^{- 1}}\end{matrix} \right. & {{Eq}.(2)}\end{matrix}$ $\begin{matrix}\left\{ \begin{matrix}{\varepsilon_{z} = {{p\varepsilon_{m}} + {\left( {1 - p} \right)\varepsilon_{d}}}} \\{\varepsilon_{x} = {\varepsilon_{y} = {\varepsilon_{d}\frac{{\left( {1 + p} \right)\varepsilon_{m}} + {\left( {1 - p} \right)\varepsilon_{d}}}{{\left( {1 - p} \right)\varepsilon_{m}} + {\left( {1 + p} \right)\varepsilon_{d}}}}}}\end{matrix} \right. & {{Eq}.(3)}\end{matrix}$

The effective material properties of the metamaterial are determined byselecting appropriate metal-dielectric combinations, filling ratios andworking wavelengths. HMMs get their name from the topology of theisofrequency surface. In vacuum, the linear dispersion and isotropicbehavior of propagating waves implies a spherical isofrequency surfacegiven by the equation k_(x) ²+k_(y) ²+k_(z) ²=ω²/c². Here, ω is thefrequency of radiation and c is the velocity of light in free space. Foranisotropic metamaterials, this isofrequency relation changes to (k_(x)²+k_(y) ²)/ϵ_(z)+k_(z) ²/ϵ_(x)=ω²/c², where the in-plane isotropiccomponents are ϵ_(x)=ϵ_(y) and the out of plane component is ϵ_(z). Ifϵ_(x) and ϵ_(z) have different signs, the isofrequency surface of themetamaterial becomes hyperbolic, as shown in FIG. 1(D) (ϵ_(x)>0,ϵ_(z)<0) and FIG. 1(E) (ϵ_(x)<0, ϵ_(z)>0). The high transverse k-vectorcoverage is the underlying foundation for light control at nanoscaledimensions using HMMs. Since the HMMs are very dispersive, and highlygeometry dependent, the working wavelength ranges for multilayer andnanowire systems are UV-visible and visible-IR, respectively. Therefore,appropriate structure/material systems first need to be selected basedon the wavelength region required for a given application then themetamaterial can be engineered to achieve the dispersion relation ofwaves that provides unique electromagnetic modes. Some of the uniqueapplications for these types of HMMs include hyperlenses, negativerefraction, sub-wavelength modes, and spontaneous emission engineering.

The present document discloses a hyperbolic metamaterial particles (HMP)and techniques for manufacturing the material. In some embodiments, theHMP is a material comprised of alternating layers of dielectricmaterials and materials that possess large enough carrier concentrationsto support plasmons at the operating wavelengths (plasmonic materials).Two examples of this are the hyperbolic tubes and spheres as shown inFIGS. 2(A) and 2(B). In this patent document, the plasmonic materialsmay be implemented by semiconductors that contain more free electronsthan the pure semiconductors. For example, the plasmonic materials mayinclude semiconductors doped with metal impurities.

FIGS. 2(A) and 2(B) illustrate two examples of hyperbolic metamaterialparticles in the form of (A) hyperbolic spheres and (B) hyperbolictubes. As can be seen here, the hyperbolic metamaterial particlesimplemented based on the disclosed technology may be made of alternatinglayers 100 and 110 of dielectric and plasmonic layers. In an embodiment,examples of the plasmonic layers may include semiconductor layers dopedwith impurities. For example, the impurities may include metalimpurities. Examples of the plasmonic layers may include TransparentConduction Oxide (TCO) layers, Doped Oxide layers, Aluminum Zinc Oxide(AZO) layers, Indium Tin Oxide (ITO) layers, and Titanium Nitride (TiN)layers.

FIG. 3 is an example schematic of coupled HNTs. As shown in FIG. 3 , ahyperbolic nanotube can be defined in terms of its length, L, totaldiameter, D_(t), and air core diameter, D_(c). The shell is comprised ofn HMM periods with alternating plasmonic (e.g., metal, semiconductordoped with impurities) and dielectric concentric layers of thicknessest_(m) and t_(d), respectively. Here, one period has a thickness, p,equal to t_(m)+t_(d) and the total shell thickness is given as theproduct of n and p. When oriented in an array, the lattice period, a,and HNT gap, g, are also defined, where a=g+D_(t). For example, theparameters L=1.5 μm, D_(t)=740 nm, D_(c)=220 nm, g=30 nm, a=770 nm,n=13, and t_(m)=t_(d)=10 nm may be used. The origin of broadbandabsorption is theoretically examined by finite difference time domainmeasurements using hyperbolic nanotubes comprised of zinc oxide as thedielectric and aluminum doped zinc oxide as the plasmonic material.

In some embodiments, the length (height) L of the hyperbolic nanotube isin a range of from about 100 nm to about 50 μm, the total diameter D_(t)of the hyperbolic nanotube is in a range of from about 200 nm to about 5μm, the air core diameter D_(c) is in a range of from about 50 nm toabout 2 μm, the number ‘n’ of HMM periods is in a range of from about 2to about 100, the thickness t_(m) of the alternating plasmonic layer isin a range of from about 5 nm to about 200 nm, the thickness t_(d) ofthe alternating dielectric layer is in a range of from about 5 nm toabout 200 nm, the thickness ‘p’ of one period is in a range of fromabout 10 nm to about 400 nm, the lattice period (e.g., a distancebetween centers of adjacent HMMS) ‘a’ is in a range of from about 100 nmto about 20 μm, the HNT gap ‘g’ is in a range of from about 0 to about10 μm, and the deposition temperature of the HNT arrays is in a range offrom about 100° C. to about 300° C.

FIGS. 4(A) and 4(B) show simulated absorption of (A) the exact structureand an EMA to describe the HNT shell and (B) EMA HNT arrays with aircores and solid hyperbolic metamaterial nanowires. Using complexpermittivity values, the simulated absorption of a closely packed HNTarray is shown in FIG. 4(A). The parameters used in the simulations areL=1.5 μm, Dt=740 nm, Dc=220 nm, g=30 nm, a=770 nm, n=13, and tm=td=10nm. The data show absorption values greater than 90% from 1,400 to 2,000nm. In addition to simulations that explicitly compute for a structuredescribed by each individual ZnO and AZO layer (exact structure),simulations may be carried out where the permittivity of the shellmaterial is replaced with the permittivity tensor derived from EMA forcurved multilayer structures. This is accomplished by describing theelectrical response of the system in terms of a diagonal effectivepermittivity tensor:

$\begin{matrix}{\varepsilon^{eff} = \begin{bmatrix}\varepsilon_{xx} & 0 & 0 \\0 & \varepsilon_{yy} & 0 \\0 & 0 & \varepsilon_{zz}\end{bmatrix}} & {{Eq}.(4)}\end{matrix}$

where the elements ϵ_(xx), ϵ_(yy), and ϵ_(zz) describe the permittivityalong the three principal directions of a Cartesian (rectangular)coordinate system.

Hyperbolic dispersion arises when one of the elements is unique and ofopposite sign to the remaining two that are of equal value:

ϵ_(xx)=ϵ_(∥)>0; ϵ_(yy)=ϵ_(zz)=ϵ_(⊥)<0   Eq. (5)

or

ϵ_(xx)=ϵ_(∥)<0; ϵ_(yy)=ϵ_(zz)=ϵ_(⊥)>0   Eq. (6)

where the parallel and perpendicular directions are referenced withrespect to the interfaces between constituent materials.

For a planar multilayer HMM made up of oscillating layers of metal anddielectric materials, the tensor elements can be given as:

$\begin{matrix}{\varepsilon_{\bot} = {{{ff}_{m}\varepsilon_{m}} + {\left( {1 - {ff}_{m}} \right)\varepsilon_{d}}}} & {{Eq}.(7)}\end{matrix}$ $\begin{matrix}{\varepsilon_{\parallel} = \left( {\frac{{ff}_{m}}{\varepsilon_{m}} + \frac{1 - {ff}_{m}}{\varepsilon_{d}}} \right)^{- 1}} & {{Eq}.(8)}\end{matrix}$

where ff_(m) is the filling fraction of the metallic component, andϵ_(m) and ϵ_(d) are the permittivities of the metal and dielectric,respectively.

A cylindrical multilayer, with alternating layers forming concentriccircles, can be accurately described using a coordinate transformation.In Cartesian coordinates, the effective permittivity tensor of thecylindrical multilayer is:

$\begin{matrix}{\varepsilon^{eff} = \begin{bmatrix}{{\varepsilon_{\parallel}{\cos^{2}(\phi)}} + {\varepsilon_{\bot}{\sin^{2}(\phi)}}} & {\left( {\varepsilon_{\parallel} - \varepsilon_{\bot}} \right){\sin(\phi)}{\cos(\phi)}} & 0 \\{\left( {\varepsilon_{\parallel} - \varepsilon_{\bot}} \right){\sin(\phi)}{\cos(\phi)}} & {{\varepsilon_{\parallel}{\sin^{2}(\phi)}} + {\varepsilon_{\bot}{\cos^{2}(\phi)}}} & 0 \\0 & 0 & \varepsilon_{\bot}\end{bmatrix}} & {{Eq}.(9)}\end{matrix}$

where the angle ϕ describes rotation about the axis of symmetry.

As shown in FIG. 4(A), the exact structure agrees well with EMA. Thus,it is possible to simulate the structure with less computational expenseand study the effects of adjusting Dt and Dc without changing n and p.In FIG. 4(B), the simulations also show that the existence of the corehas little effect on the absorption.

FIG. 5(A) shows examples of simulated absorption of HNTs with increasingn from 1-8 and 13 with a constant inter-pillar gap distance. As ndecreases from 8 to 1 with constant g, the broadband absorptionseparates into two peaks at 1.45 μm and somewhere between 2-3 μm. FIG.5(B) shows simulated absorption of exact structure HNT arrays withvarying a and constant Dt. FIG. 5(C) shows simulated absorption of EMAsolid hyperbolic nanowire arrays of varying diameter. FIGS. 5(D) and5(E) show simulated absorption using the exact structure of HNTs withvarying (D) lengths and (E) ffm in the shell. FIG. 5(F) shows simulatedabsorption using the exact structure of common HMMs compared with theHNTs.

Broadband absorption occurs when there are a sufficient amount ofperiods (>˜6) within an HMMP as shown in FIG. 5(A) and when theparticles are closely packed. This is due to the coalescence ofabsorption arising from the coalescence of surface/bulk plasmons andlossy hyperbolic modes. The lossy hyperbolic modes occur at longerwavelengths since only the longer wavelengths can be tuned by adjustingthe filling fraction, ffm, of the plasmonic material.

In addition, the shorter wavelength peak is held constant at 1.4 μmwhich corresponds to the epsilon zero crossing wavelength of theplasmonic material and is therefore attributed to surface/bulk plasmons.Longer wavelength absorption is due to scattering from surfaceroughness.

Existing absorbers are either thick, non-transferrable and/ornon-selective. The absorbers described herein can operate withinrelatively wide bands (>600 nm) with over 90% absorption and maxabsorptions of above 98%. The materials created have high absorption ofboth transverse-electric (TE) and transverse-magnetic (TM) modes atangles greater than 50 ° . The material system can also be chosen towithstand high temperatures and corrosive environments. Further, thematerials are comprised of particles allowing them to be transferred andoperate on any substrate. This allows for greater control overselectivity and flexibility.

As mentioned above, the broadband absorption is due to the coalescenceof multiple absorption peaks. As shown in FIG. 5(A), as n decreases from8 to 1 with constant g, the broadband absorption separates into twopeaks at 1.45 μm and about 2-3 μm. Moreover, when n and Dt are heldconstant, but a is increased from 770 nm to 3500 nm, a deconvolution ofthe peaks may be observed as shown in FIG. 5(B). The effects of varyingDt can be demonstrated by using the EMA to eliminate the effect of n andreplace the air core with the effective medium. Using solid HMMnanowires, no significant effect on the absorption band is observed whenDt is varied from 740 nm to 260 nm with constant g as shown in FIG.5(C). Simulations also show that the length of the HNTs does notcontribute much to the absorption when greater than 1.5 μm. However, atlengths smaller than 1.0 μm the absorption decreases due to increasedtransmission.

Broadband absorption is achieved when there is a sufficient number ofperiods and when the HNTs are packed close enough to allow strongresonance coupling. Physically, g is responsible for the intercouplingof the HNTs and n is responsible for the number of higher orderplasmon-polaritions modes supported by the HMM. The bandwidth of highwavevector modes supported by the HMM depends upon p but not n. As nincreases, however, the discrete modes supported by the HMM maycoalesce. Due to the high losses, the discreteness vanishes and a singleabsorption band appears. The shorter wavelength peak located at 1450 nmcorresponds to the zero point (cross-over wavelength) in the realpermittivity of AZO and shows little tunability. Therefore, the shortwavelength peak arises due to excitation of bulk and/or surfaceplasmon-polaritons in the AZO. Conversely, the longer wavelength peakintensifies as n gets larger as shown in FIG. 5(A). Importantly, theaccuracy of EMA increases with a larger number of periods and only thelong wavelength peak changes with the AZO filling fraction, ffm.Additionally, as air gaps are introduced into the HMM, the systemevolves into a photonic hypercrystal which supports additional modes andallows for additional absorption band tailoring.

The mechanism for absorption can be better appreciated by examining thedispersion of the AZO/ZnO/air system analytically. Absorption may occurthrough the excitation of bulk plasmon polaritions that are supported bythe periodic AZO/ZnO/air system. Due to the high losses, all excitedmodes may eventually dissipate, contributing to absorption. Therefore,absorption occurs at frequencies for which the system supports modes.Because material system is highly dispersive, traditional methods forcomputing the entire 2D band structure, such as the plane wave expansionmethod or FDTD, are unreliable and inefficient. To address thiscomplication, a number of simplifying assumptions may be made. Namely,we ignore losses and approximate the entire structure as a 1D photonichypercrystal with variable air gap, g. While ignoring the center airhole and excitation of modes traveling along the length of the HNT, ourassumptions provide a first-order description of the actual system andyields results qualitatively consistent with the experimental absorptionspectra.

Propagation of electromagnetic waves through periodic media isconveniently described using Bloch's Theorem. Because the layerthicknesses of the AZO/ZnO HMM are deeply subwavelength, effectivemedium theory may be used to treat the HMM as a homogeneous uniaxiallayer. The general formalism for wave propagation through periodicanisotropic layers for arbitrary angles of incidence and arbitraryorientation of optical axes is rather complex and necessitates numericalsolution of a 4×4 matrix problem with complex coefficients. However, forthe case of a periodic system consisting of one uniaxial and oneisotropic layer, with the optical axis of the uniaxial layer alignedwith the optical axis of the bilayer, the problem resembles anisotropic-isotropic periodic system with an angle-dependent permittivityincluded in the dispersion relation for TM waves.

The wave-vector component parallel to the optical axis of the system,for both TM and TE polarizations may be determined as a function of thetransverse wave-vector component, k⊥, and vacuum wavelength, λ0. Thedispersion relations of a 1D infinitely periodic uniaxial-isotropicsystem, with period length, a, are:

$\begin{matrix}{{k_{\parallel {,{TE}}}\left( {\lambda_{0},k_{\bot}} \right)} = {\frac{1}{a}{\cos^{- 1}\left( \frac{A_{TE} + D_{TE}}{2} \right)}}} & {{Eq}.(10)}\end{matrix}$ $\begin{matrix}{{k_{\parallel {,{TM}}}\left( {\lambda_{0},k_{\bot}} \right)} = {\frac{1}{a}{\cos^{- 1}\left( \frac{A_{TM} + D_{TM}}{2} \right)}}} & {{Eq}.(11)}\end{matrix}$ where $\begin{matrix}{A_{TE} = {{\exp\left( {ik_{\parallel {,{{TE}(H)}}}d_{H}} \right)}\left\lbrack {{\cos\left( {k_{\parallel {,0}}d_{0}} \right)} + {\frac{i}{2}\left( {\frac{k_{\parallel {,0}}}{k_{\parallel {,{{TE}(H)}}}} + \frac{k_{\parallel {,{{TE}(H)}}}}{k_{\parallel {,0}}}} \right){\sin\left( {k_{\parallel {,0}}d_{0}} \right)}}} \right\rbrack}} & {{Eq}.(12)}\end{matrix}$ $\begin{matrix}{D_{TE} = {{\exp\left( {ik_{\parallel {,{{TE}(H)}}}d_{H}} \right)}\left\lbrack {{\cos\left( {k_{\parallel {,0}}d_{0}} \right)} - {\frac{i}{2}\left( {\frac{k_{\parallel {,0}}}{k_{\parallel {,{{TE}(H)}}}} + \frac{k_{\parallel {,{{TE}(H)}}}}{k_{\parallel {,0}}}} \right){\sin\left( {k_{\parallel {,0}}d_{0}} \right)}}} \right\rbrack}} & {{Eq}.(13)}\end{matrix}$ $\begin{matrix}{A_{TM} = {{\exp\left( {ik_{\parallel {,{{TM}(H)}}}d_{H}} \right)}{\left\lbrack {{\cos\left( {k_{\parallel {,0}}d_{0}} \right)} + {\frac{i}{2}\left( {\frac{{\varepsilon_{T{M(H)}}(\theta)}k_{{\parallel {,0}})}}{\varepsilon_{air}k_{\parallel {,{{TM}(H)}}}} + \frac{\varepsilon_{air}k_{\parallel {{TM}(H)}}}{{\varepsilon_{T{M(H)}}(\theta)}k_{\parallel {,0}}}} \right){\sin\left( {k_{\parallel {,0}}d_{0}} \right)}}} \right\rbrack}}} & {{Eq}.(14)}\end{matrix}$ $\begin{matrix}{D_{TM} = {{\exp\left( {{- i}k_{\parallel {,{{TM}(H)}}}d_{H}} \right)}{\left\lbrack {{\cos\left( {k_{\parallel {,0}}d_{0}} \right)} - {\frac{i}{2}\left( {\frac{{\varepsilon_{T{M(H)}}(\theta)}k_{{\parallel {,0}})}}{\varepsilon_{air}k_{\parallel {,{{TM}(H)}}}} + \frac{\varepsilon_{air}k_{\parallel {,{{TM}(H)}}}}{{\varepsilon_{T{M(H)}}(\theta)}k_{\parallel {,0}}}} \right){\sin\left( {k_{\parallel {,0}}d_{0}} \right)}}} \right\rbrack}}} & {{Eq}.(15)}\end{matrix}$

In equations (12)-(15), the longitudinal wave-vector components are:

$\begin{matrix}{k_{\parallel {,{{TE}(H)}}} = \sqrt{{\varepsilon_{\bot}k_{0}^{2}} - k_{\bot}^{2}}} & {{Eq}.(16)}\end{matrix}$ $\begin{matrix}{k_{\parallel {,{{TM}(H)}}} = \sqrt{\varepsilon_{\bot}\left( {k_{0}^{2} - \frac{k_{\bot}^{2}}{\varepsilon_{\parallel}}} \right)}} & {{Eq}.(17)}\end{matrix}$

with k0=2π/λ0. The effective permittivity elements, ϵ_⊥ and ϵ_∥, aregiven by eqns. 4 and 5 and k⊥ is an independent, purely real-valuedvariable. Also, ϵ_(air)=1 is the permittivity of air, dH and d0 are thelengths of the uniaxial HMM and isotropic air layers, respectively, andk_(∥,0)=(k_0{circumflex over ( )}2-k_⊥{circumflex over ( )}2){circumflexover ( )}(1/2). The photonic hypercrystal (PhHC) described here has aperiod of a=dH+d0. Finally, the angle-dependent permittivity element ofequations (14)-(15) are defined as:

$\begin{matrix}{{\varepsilon_{T{M(H)}}(\theta)} = \sqrt{{\varepsilon_{\parallel}^{2}\cos^{2}\theta} + {\varepsilon_{\bot}^{2}\sin^{2}\theta}}} & {{Eq}.(18)}\end{matrix}$ where, $\begin{matrix}{\theta \equiv {\cot^{- 1}\left( \frac{k_{\parallel {,{{TM}(H)}}}}{k_{\bot}} \right)}} & {{Eq}.(19)}\end{matrix}$

For waves propagating normal to the layer interfaces, θ=90°, the TM andTE modes are degenerate.

FIG. 6 shows effective material dispersion. Equations 7 and 8 areAZO-ZnO uniaxial multilayer HMM with ffm=0.5. Upper inset shows thestructure with the definition of parallel and normal directions withrespect to the optical axis of the multilayer that is homogenized. Lowerinset is a magnified plot of the transition between type I and type IIhyperbolic dispersion for the TM modes. The structure implemented basedon the disclosed technology is shown schematically in FIG. 6 . Alsoshown is the effective permittivity elements of the AZO/ZnO HMM paralleland normal to optical axis of the system, which is normal to the layerinterfaces. All calculations here use a constant metallic fillingfraction, ffm=0.5. From effective medium theory, the HMM exhibits type-Ihyperbolic dispersion for 1450 nm<λ0<1720 nm and type-II hyperbolicdispersion for λ0>1750 nm for TM modes, i.e., modes with an electricfield component normal to the layer interfaces. Dispersion is ellipticalfor λ0<1450 nm for both TM and TE modes. In the type-I and type-IIhyperbolic regimes, TE modes are elliptical and not supported,respectively.

As a increases with a fixed dH, the fill fraction of the HMM in the PhHCbecomes smaller. This corresponds to the experimental situation of adecreasing hyperbolic nanotube fill fraction, ff_(HNT), which we defineas:

$\begin{matrix}{{ff}_{HNT} = {\frac{\pi}{6\sqrt{3}}\frac{D_{t}^{2}}{a^{2}}}} & {{Eq}.(20)}\end{matrix}$

Compared to PhHCs with a=1500 nm, a=2500 nm, and a=3500 nm, a PhHC witha=770 nm has a ffHNT 4x, 11x, and 21x greater, respectively. Therefore,a naive prediction would assume that the close-packed PhHC with a=770 nmhas 4x, 11x, and 21x greater absorption at a given wavelength which isnot observed in the numerical simulations displayed in FIG. 4(B).

FIG. 7 compares a PhHC of constant dH=740 nm and a increasing from 740nm to 3500 nm as a function of both λ0 and k⊥. Here, the existence oftype-I and type-II hyperbolic dispersion may be observed. Stop bands inthe PhHC may also be observed as a increases in the type I regime fork™/k₀<1 and in the type II regime for k⊥/k₀>1. Pass bands in the PhHCmay also be observed as a increases in the type II regime for k⊥/k₀<1.

Despite the appearance of bandgaps during the formation of the PhHC,additional modes also appear. These modes are responsible for thelarger-than-expected absorption of the PhHC with large a, particularlyin the type-II regime. The prediction based on HNT density does notaccount for the appearance of additional modes as the HMM evolves in thePhHC. These features correlate well with the numerical simulationpresented in FIG. 5(B).

Based on these results, it is clear that the absorption spectrum dependsupon both the wavelength-scale periodicity of the hypercrystal and thedeeply sub-wavelength scale of the HMM. The simultaneous control ofthese structural parameters enables a tailorable absorption spectrumunachievable in other systems, such as self-aligned carbon nanotubes.Therefore, the near-perfectly absorbing photonic hypercrystal representsa new platform for tailor-made manipulation of light. While we havedemonstrated this effect using AZO, ZnO, and air in the near-infrared,we stress that our methodology is general, and may be applied to myriadmaterial systems across the electromagnetic spectrum.

FIGS. 8(A) and 8(B) show SEM images of the HNT arrays as viewed in theplane (a) parallel and (b) perpendicular to the nanotube axis. As can beseen here, the HNT array implemented based on the disclosed technologymay have a closely packed triangular geometry.

Broadband and selective absorption has been experimentally demonstratedfor the AZO/ZnO hyperbolic tubes made by depositing AZO/ZnO HMM filmsgrown by atomic layer deposition. Absorption over 90% is observed from1400-2400 nm with a maximum absorption >98% at 1550 nm for hyperbolicnanotubes whereas solid AZO nanotubes are shown to have less absorptionwithin a smaller band. The absorption band can also be tuned by changingparameters that can tune the carrier concentration of the plasmonicmaterial. This is also demonstrated for AZO/ZnO hyperbolic tubes wherethe atomic layer deposition reactor temperature is adjusted from 200° C.to 185° C. to adjust the absorption window.

The hyperbolic tubes also show a wide absorption angle as shown in FIGS.9(B) and 8(C). FIG. 9(A) shows absorption spectra of HNT arraysdeposited at a temperature of 185° C. and 200 ° C. along with a spectrumof a pure AZO nanotube array deposited at 200° C. FIG. 9(B) shows aschematic of the incident radiation at angle θ showing TM polarization.Wide-angle absorption spectra are shown in FIGS. 9(C) and 9(D) for (C)TM and (D) TE polarizations of an

HNT array deposited at 200° C. The color corresponds to the percentabsorption.

Broadband absorption of above 87% from 1,200 nm to over 2,200 nm isdemonstrated with a maximum absorption of 98.1% at 1,550 nm for a Td of200° C. For a Td of 185° C., broadband absorption of above 87% isobserved from 1,500 nm to over 2,200 nm with a maximum absorption of96.3% at 1,700 nm. As a control, pure AZO nanotubes are also plotted,which exhibit only a modest absorption maximum of 82.5% at 1,400 nm witha narrow spectral width. As can be seen in FIG. 9(A). The HNTs showexcellent selectivity by displaying increased reflection at longerwavelengths. This is due to an increased impedance mismatch as the realpermittivity of AZO becomes increasingly negative at longer wavelengths.The angular dependence may be demonstrated by sputtering a 100-nm goldfilm on the back of the silicon. This may allow only the specularreflectance to be considered when calculating the absorption[absorption(%)=100%−reflectance(%)]. This method may be valid within 1%from 1,500 to 2,400 nm and within 6% from 1,400 to 1,500 nm.

As shown in FIGS. 9(C) and 9(D), the HNTs possess omnidirectionalabsorption for both TE and TM polarizations. For TM polarization, theabsorption is maintained above 85% at all angles and wavelengthsmeasured. For TE polarization, the absorption is diminished at higherangles and longer wavelengths. However, an absorption of >70% ismaintained across the measured wavelengths and angles smaller than 50°.

FIG. 10(A) shows a comparison of experimental and simulated absorptionspectra for the sample A of Table 1 below. FIGS. 10(B) and 10(C) showexamples of transmission and absorption spectra for samples A-C withparameters as described in Table 1 after flexing several times as shownin FIG. 10(E). FIGS. 10(D) and 10(E) are photographs of the sample Ashowing visible transparency (D) and mechanical flexibility (E). FIG.10(F) shows an example SEM image of an HNT array (Sample B of Table 1)after transferring to a flexible PDMS substrate and flexing severaltimes as shown in FIG. 10(E).

The particles can also be transferred to a more desirable substrate.This is demonstrated in FIGS. 10(A)-10(F) where the hyperbolic tubesdeposited are transferred to a polydimethylsiloxane (PDMS) substratewith parameters described in Table 1.

The HNT arrays implemented based on the disclosed technology operate asa standalone material even after being removed from the growthsubstrate. The arrays may be transferred to a thin, flexible, andvisibly transparent substrate by spin coating 60 μm ofpolydimethylsiloxane (PDMS) onto the top of the arrays and removing thesilicon via reactive ion etching (ME). This may be carried out for threedifferent arrays with various parameters as described in Table 1 belowand observed from direct imaging. Referring to FIG. 10(A), theabsorption spectra of sample A and a simulated exact structure (with asimilar structure to sample A, but with a rounded top to replicateexperimental conditions) have excellent agreement. This suggests thatthe shorter wavelength discrepancy between the simulated andexperimental absorption described earlier is due to the siliconsubstrate either from reflection back into the HNTs or absorption of thetextured surface created during the HNT fabrication. FIGS. 10(B) and9(C) illustrate the transmission and absorption spectra of the threesamples to show that a large transmission window is observed from 500 to1,300 nm, with the broadest IR absorption band belonging to sample A atthe expense of a narrower and weaker transmission window. Thetransmission can be slightly increased by lowering Td and keeping thegeometry constant, but the largest gain in transmission is observed whenL is decreased and g is increased (sample C). Because all of theconstituent materials (AZO, ZnO, and PDMS) in the free-standing HNTarray have large optical transparencies throughout the visible, andreflection accounts for below 12% of the decreased transparency, theadditional absorption stems from silicon contaminates left over from thetransfer process and/or subband levels (defects states) in the ZnO/AZOlayers. With further refinement in the synthesis, etching, andtransferring steps, it is anticipated that a significant improvement canbe made in the visible transparency. After transferring the HNT array tothe PDMS film, they can be reproducibly flexed while still maintainingtheir excellent broadband absorption in the near IR (NIR), andtransmission in visible, as well as their physical interface with thepolymer (FIGS. 10(D)-10(F)).

TABLE 1 Parameters of Samples A-C L D_(t) D_(c) t_(m) t_(d) p a g T_(d)(μm) (nm) (nm) n (nm) (nm) (nm) (nm) (nm) (° C.) A 2.05 700 220 12 10 1020 770  70 200 B 2.05 700 220 12 10 10 20 770  70 185 C 1.39 620 140 1210 10 20 770 150 185

Here, the absorption is well simulated by finite-difference time domain(FDTD) simulations as shown in FIG. 10(A). The transmission window isobserved from 500 nm to 1300 nm as shown in FIG. 10(B). Further,experimental absorption is still maintained above 90% attelecommunication wavelengths as shown in FIG. 10(C).

FIG. 11 shows an example infrared (IR) reflection spectrum of an HNTarray. It can be seen that selectivity is observed by increasedreflection at wavelengths over 2 μm.

FIGS. 12(A) and 12(B) show an example of a simulated absorption spectraof a 5 closely packed TiN/(Al,Sc)N based HNT array as shown in FIG.12(A) and hyperbolic nanosphere array (FIG. 12(B)). The HNT arrayparameters L, Dt, Dc, g, a, n, tm and td used in the simulation were 750nm, 500 nm, 60 nm, 30 nm, 530 nm, 11, 10 nm, and 10 nm, respectively.The Dt, Dc, g, a, n, tm, and td used for the hyperbolic nanospheresimulation were 540 nm, 60 nm, 30 nm, 570 nm, 12, 10 nm and 10 nm,respectively.

Simulations also reveal that these materials can use alternativematerials to operate in other spectra such as the visible. Beyond theAZO/ZnO HNT system, selective and broadband absorption can be applied tomany other material systems and geometries. For example, theTiN/(Al,Sc)N material system may produce high-quality epitaxial HMIswith refractory qualities. This is demonstrated with the TiN/(Al,Sc)Nsystem for both the nanotube and nanosphere geometries. As seen in FIGS.12(A) and 12(B), these materials can provide over 90% absorptionthroughout the entire visible spectrum.

THMMP arrays with these materials could be advantageous for applicationssuch as thermophotovoltaics (TPV) and visible emitters that require manycycles at elevated temperatures (>1,000° C.). Some existing work in thisarea focuses on nonselective absorbers and plasmonic metamaterial. Theseplanar materials, however, either lose efficiency due to parasiticradiation at higher temperatures or can crack and delaminate due tothermal expansion mismatch between the absorber layer and substrate. Amajor advantage of a THMMP array is that it has a reduced substratecontact area that minimizes the impact of thermal expansion mismatchesand would allow for a wider selection of substrates to be used whilepromoting the use of facile transfer techniques. Simulations reveal thatTiN/(Al,Sc)N based HNT arrays (L=750 nm, Dt=500 nm, Dc=30 nm, n=11,tm=td=10 nm, a=530 nm) have absorption values of above 93% throughoutthe entire visible spectrum while suppressing absorption at longerwavelengths as shown in FIG. 12(A). In comparison with the HNT geometry,closely packed hyperbolic nanospheres may be particularly useful becausethey are expected to have less angular dependence and can be depositedonto substrates via low-cost/scalable processes such as nanoparticleself-assembly and spray-on techniques. Simulations also reveal thatthese types of arrays made from a TiN/(Al,Sc)N material system have alarge absorption (e.g., above 89% throughout the entire visiblespectrum) while suppressing absorption in the NIR with only a540-nm-thick single monolayer as shown in FIG. 12(B). In addition,potential uses of the material include thermal/visible emitters,absorbers/heat exchangers, thermophotovoltaics and camouflage material.

In some embodiments, a layered metamaterial structure comprising a firstlayer including a dielectric material and a second layer including aplasmonic material may be used for the described application. Theplasmonic material has a carrier concentration that exhibits plasmons ina target operating frequency band. In some embodiments, the structuremay be an array that includes spherical or cylindrical nanoparticles.The structure may include five or more alternating layers of AZO and ZnOor other similar compounds exhibiting dielectric and/or plasmonicproperties. Here, the plasmonic properties may be exhibited in a targetoperating frequency band including 1,550 nm wavelength. In anembodiment, the structure comprises one or more spherical nanoparticlesin which a number of concentric first and second layers alternate witheach other. In another embodiment, the structure comprises one or morecylindrical nanotubes comprising coaxial layering of the first layer andthe second layer along a longitudinal axis. Here, the structure mayfurther include an air core at the longitudinal axis. In an embodiment,the structure may have a dimension that is between 200 nm and 1micrometer.

In some embodiments, a method of fabricating an array of nanoparticlestructures includes generating a silicon hole array template usingnanosphere lithography, filling in holes of the silicon hole array usingatomic layer deposition by multilayered structures of hyperbolicmetamaterials (HMIs), etching, using a reactive ion etching process, atop layer of result of the filling operation, thereby exposingunderlying silicon of the silicon hole array, and removing siliconsurrounding hyperbolic metamaterial to leave behind an array of packedmultilayered structures of HMMs. Here, the multilayered structures ofHMMs include a plurality of spherical nanoparticles in which a number oflayers of concentric aluminum zinc oxide (AZO) and zinc oxide (ZnO)alternate with each other. The multilayered structures of HMMs mayinclude at least five layers. Here, in fabricating the nanoparticlestructures, Transparent Conduction Oxide (TCO), Doped Oxide layers,Aluminum Zinc Oxide (AZO), Indium Tin Oxide (ITO), or Titanium Nitride(TiN) may be used.

FIGS. 13(A) to 13(I) are cross-section diagrams that show an example HNTfabrication process. In some embodiments, HNT arrays may be fabricatedusing silicon nanohole templates created by nanosphere lithography. InFIG. 13(A), a silicon dioxide (SIO₂) sphere 810 is formed on a substrate(e.g., silicon substrate) 800. For example, Stober particles (770-nmdiameter) may be grown from standard processes and used as a nanospheremask. Next, the Stober particles may be functionalized with3-aminopropylmethyldiethoxysilane (APMDES), cleaned, and deposited ontop-type, double-sided polished, prime silicon using a Langmuir Blodgetttrough. In FIG. 13(B), a reactive ion etching (RIE) process may beperformed using CHF3 and Ar gases to reduce the size of the silicondioxide particles 810 formed on the substrate 800 to a desired size of asilicon dioxide sphere 815. In FIG. 13(C), a nickel film 820 may bedeposited on the resulting structure using an electron beam evaporator.For example, 45-nm-thick nickel film may be deposited. In FIG. 13(D),the silicon dioxide spheres 815 are chemically or mechanically removed.In FIG. 13(E), using SF6 and C4F8 gases, the exposed silicon may beetched to create the nanohole template. After removing the nickel maskusing TFB nickel etchant as shown in FIG. 13(F), the ZnO/AZO HMM may bedeposited into the nanoholes with an atomic layer deposition (ALD)system operating at a reactor temperature of, e.g., 185 or 200 ° C., asshown in FIG. 13(G). In an embodiment, diethylzinc, trimethylaluminum,and deionized water may be used as the zinc, aluminum, and oxygensources, respectively. For example, the Zn:Al pulse ratio for the AZOfilms may be kept constant at 20:1 for all experiments. The filmthicknesses may be first determined for each material on planar siliconsubstrates by ellipsometry at each deposition temperature. In anembodiment, a total HMM thickness of about 350 nm may be deposited onthe nanohole template such that the nanoholes are completely sealed.Here, each individual ZnO and AZO layer may be, e.g., 10 nm. Next, a20-nm protection layer of HfO2 may be deposited via ALD by usingtetrakis (dimethylamido) hafnium(IV) and water as the hafnium and oxygensources, respectively. After depositing the HMM and protection layer,rapid thermal processing may be carried out to activate the aluminumdopants using a thermal processing equipment with a 1.5 L·min-1 nitrogenflow rate. In FIG. 13(H), the top layers of HfO2 and ZnO/AZO may beremoved, e.g., using reactive ion etching (RIE) with CHF3 and Ar gasesuntil the top silicon was exposed. In FIG. 13(I). the exposed siliconmay be removed using an etching equipment with SF6 and C4F8 gases,leaving behind the HNT array. The roughened HNTs may be created byadditional etching of the HNTs (deposited at a temperature of 200° C.)using RIE with CHF3 and Ar gases. Before measurements, dilute HCl may beapplied to the back of the resultant structure to remove anycontaminates that may have been generated during the ALD process.

The HNT arrays may be transferred secondary substrates, such as visiblytransparent and mechanically flexible substrates, while maintainingtheir broadband absorption in the NIR. For example, the HNT arrays maybe transferred to a flexible polymer film by depositing a 60-μm film ofPDMS and etching away the substrate. For example, the PDMS film may becoated on the top of the HNT array by spin coating at 1,000 rpm for 45seconds and curing at 125 ° C. for 8 minutes. The etching may beperformed using xenon difluoride. The back silicon may then be cleanedwith O2 plasma and removed using an etching equipment.

FIGS. 14(A)-14(D) illustrate (a) scanning electron micrograph of a Sinanopillar array coated with a 56 nm AZO layer deposited via ALD, (b)cross-sectional image of a single Si nanopillar with an AZO coating, (c)IR extinction spectra of AZO-coated Si nanopillars at a constantdeposition temperature (200° C.) but varying Al/Zn compositions, (d)log-log plot of carrier concentration vs. resonant wavelength forvarious AZO-coated Si nanopillar arrays, where the dashed line is thelinear relationship expected from a Drude model.

To date, the only synthetic approaches that have been able to fabricateAZO materials with plasmonic responses in the NIR to short-wave IRregions (SWIR) (λ=1.4 to 3 μm) are solution grown nanoparticles and thinfilms created by pulsed laser deposition (PLD). The nanoparticlestrategies have several advantages over PLD-based deposition,specifically in their synthetic simplicity and scalability, but creatingmore complex shapes and geometries remains a major challenge for boththe nanoparticle systems and PLD. There is therefore a strong need todevelop novel deposition techniques that can be scaled-up and routinelycoat nanoscale features. One of the most versatile deposition techniquesto conformally coat large and small features is atomic layer deposition(ALD) which forms controlled monolayers in a systematic fashion througha series of sequential, self-limiting surface reaction. These surfacereactions are induced by chemical pulses into the growth chamber whichallow pinhole-free films to be fabricated with nanometer thicknessresolution. To tune the carrier concentration in ALD-deposited AZOfilms, one only has to control the frequency at which the aluminumchemical pulses (e.g., trimethylaluminum) are applied.

Recently we demonstrated that ALD can be used to fabricate high-qualityplasmonic AZO coatings on silicon nanopillar arrays that can be tunedacross the SWIR and NIR wavelengths. The silicon nanopillar arrays werefabricated using nanosphere lithography (NSL).

In this technique silica nanoparticles (˜200 nm diameters) aretransferred to a silicon substrate after being assembled into a closepacked geometry with a Langmuir-Blodgett (LB) trough. After shrinkingthe silica nanoparticles via reactive ion etching (RIE) to expose moreof the silicon substrate, RIE is used to etch the underlying silicon.Once the desired etch depth is attained, the silica nanoparticle maskscan be removed with a buffered oxide etch and the chip can be placed inthe ALD chamber. Various ALD recipes can be used for AZO, but it wasfound that the best zinc, aluminum and oxygen sources were diethylzinc(DEZ), trimethylaluminum (TMA), and water, respectively, and the optimaldeposition temperature ranged from 200° C. to 250° C. The aluminumconcentration can be controlled by changing the frequency of the TMAprecursor cycle. For example, a film with a ˜2 wt % Al/Zn (nominal) canbe synthesized using 1 TMA cycle for every 20 DEZ cycles. FIGS.14(A)-14(D) show some images and optical responses of the materials as afunction of aluminum doping. Initial findings showed that the lowestcrossover wavelength (ϵ′=0, where the material starts to behave like ametal) obtainable was 2160 nm for a 56 nm thick film. These films had adecent loss corresponding to ϵ″ ≈2 but still higher than films depositedby PLD which can reach values of ϵ″≈1.

FIGS. 15(A)-15(D) illustrate (a) real and (b) imaginary dielectricconstants of AZO and thermally processed AZO (825° C.) usingellipsometry. Samples are coated with a 12 nm thick HfO₂ passivationlayer, (c) IR extinction spectra of thermally processed AZO/HfO₂ (56/12nm thicknesses) after depositing on a Si nanopillar array and performingRTP treatment at different temperatures, (d) comparison of experimentaland FDTD simulation of an AZO/HfO₂ (56/12 nm thicknesses) layer coatedon a Si nanopillar array.

To improve the optical properties of the films, we found that thermalpost-processing (>500 ° C.) via rapid thermal processing (RTP) incombination with a protective oxide (e.g., HfO₂) film deposited on topof the AZO lowers the crossover wavelength and reduces the optical loss.FIGS. 15(A) and 15(B) show the real and imaginary dielectric constantsof an as-made sample with a 56 nm thick layer of AZO and a 12 nm thickHfO₂ along with the same sample after thermally processing at 825° C.There is a clear blue shift in ϵ′, reaching a crossover wavelength of1545 nm, and an improved optical loss, reaching values of ϵ″<0.5. Theseare impressive values that have never been attained with an ALD processbefore, and are competing with the best PLD films. Referring to FIG.15(C), the IR extinction spectra show how the plasmon resonance isinfluenced by the RTP treatment as well as the improvement in thequality factor of the resonance. Referring to FIG. 15(D),finite-difference time-domain (FDTD) simulations of the plasmonicresonances can also be carried out on the arrays which show excellentagreement with the experimental values.

Although these preliminary results demonstrate that ALD is an excellentdeposition technique to produce high-quality, tunable IR plasmoniccoatings, we have not reached its full potential. The performance of theTHC devices will be intimately coupled with the quality factor andlosses associated with the coatings. Therefore we aim to further improvethe ALD process by investigating various passivating (on top of the AZO)and buffer layers (between substrate and AZO). This will not onlyenhance the optical properties of the active materials, but helpelucidate the role of these layers. For example, it is believed that theHfO₂ passivation layer used in the data for FIGS. 15(A)-15(D) helpsprevent zinc evaporation from the AZO film during the RTP treatmentwhich can reduce the number of the accepting oxygen (O_(i)) defects thatultimately lowers the carrier concentration. However, an increase in thecarrier concentration (via Hall measurements) from 1.0×10²¹ cm⁻³ beforeRTP to 1.5×10²¹ cm⁻³ after RTP is observed for the passivated AZOsamples. Interestingly, this is opposite of the pure ZnO films whichshow a decrease in carrier concentration from 2.9×10¹⁹ cm⁻³ before RTPto 8.7×10¹⁸ cm⁻³ after RTP. This suggests that there is some activationprocess of the aluminum dopants. The amount of Al atoms that contributeto free electrons can be significantly lower in ALD than other synthetictechniques. Through an effective field model, it has been proposed thatthe lower free electrons is due to the unique layer-by-layerconfiguration in the ALD process which is opposite to PLD, sputtering,and other deposition methods. The close proximity of adjacent Al atomsin the model causes electronic repulsion that suppresses the donation offree electrons. It is possible that the 50% enhancement in the carrierconcentration of the AZO after RTP is coming from the diffusion of Alatoms into the ZnO layers creating more random orientations that cansupport higher free electrons, but we will systematically study this togain a complete understanding. For example, tools including small angleX-ray diffraction, high-resolution transmission electron microscopy, andelectrical characterization will be used on samples with different ALDdeposited passivation/buffer layers (e.g., Al₂O₃, HfO₂, ZnO, TiN, etc.)to correlate the structural and electrical properties with the plasmonicresponse of the coatings.

In some embodiments, a structure with multilayered metamaterialsincludes free-floating hyperbolic nanospheres that consist of concentriclayers of dielectric and metal. If a colloidal solution or ink comprisedof independent HMM nanospheres could be synthesized, this could be usedto create specific hyper-crystal coatings on virtually any substrate viaspray-on or printing technologies. In addition, there could be newsciences uncovered when extending the 2D multilayer system (e.g., tubes)to a 3D multilayer system.

FIG. 16 illustrates jumping particle ALD based on proposed fabricationscheme for 3D multilayer HMM nanospheres.

To explore the possibility of synthesizing HMM nanospheres twostrategies may be pursued including (1) substrate-supported nanoparticleALD, and (2) jumping particle ALD. In the first approach large area,close-packed monolayers of nanoparticles (e.g., 100-300 nm SiO₂) will befabricated on 4 inch wafers using either LB or evaporative assembly. Inthe latter, the strong capillary forces at a meniscus can help drive thenanoparticles into a close-packed geometry. The size of the silicananoparticles will be reduced via RIE to allow additional access of thechemical vapors around the nanoparticles. If further exposure to thesurface is needed, anisotropic etching with solutions such as KOH andisotropic dry etching will be used to slightly raise the nanoparticlesabove the bulk silicon substrate. These techniques are commonly used inthe under-etching of devices such as on-chip optical resonators. Afterdepositing metal/dielectric layers via ALD the nanoparticles will beremoved from the substrates by mildly sonicating in a solvent or bydepositing a polymer such as polyvinylbutyral (PVB), followed bysubsequent peel-off and dissolution with a solvent (e.g. ethanol).Polymers such as PVB have been shown to easily adhere to, remove, anddisperse nanoparticles attached to a substrate. If challenges arise withdetaching the nanoparticles from the substrate, sacrificial layers(e.g., photoresists, metals, etc.) deposited on the wafers prior tonanoparticle assembly will be explored. As illustrated in FIG. 16 , thesecond synthetic strategy involves the elevation of nanoparticles withinthe ALD chamber via ultrasonic perturbation. Similar nanoparticles asthose used in the substrate-supported deposition will be placed in theALD chamber which is coupled to an ultrasonic transducer or vibrationalstage. Under the right frequency and power the nanoparticles will startto jump in the chamber giving full access of the surface during the ALDcycles.

Once the HMI nanospheres are synthesized they will be dispersed in asolvent (e.g., alcohol), and their optical properties will becharacterized by depositing them on optically transparent substratessuch as glasses or polymers via self-assembly (as described above) orspay-on techniques. In particular, the angle-dependent opticalabsorption in the NIR and transparency in the visible will be evaluatedby using the AZO/ZnO metal/dielectric multilayers. Parameters such asnanoparticle size, inter-particle distance, and extent of ordering willbe correlated to the absorption properties.

Since ALD is not limited to just oxide materials, this opens up accessto other HMM systems that show different wavelength responses comparedto the AZO/ZnO structures. For example, TiN/(Al,Sc)N materials have beenshown to produce high-quality epitaxial HMMs with refractory qualitiesin the visible. Arrays with these types of materials could beadvantageous for a variety of applications including TPVs and visibleemitters that require cycling at elevated temperatures (>1000° C.).Currently there aren't any tunable and selective visible broadbandabsorbers that can satisfy these constraints. There is ongoing researchon non-selective absorbers and other plasmonic metamaterials, but theseare planar systems that either loose efficiency due to parasiticradiation at high temperature or can crack/delaminate due thermalexpansion mismatches between the active layer and substrate. Thedispensable HMM nanospheres would reduce substrate contact and allow fora wider selection of substrates to be used. To demonstrate the opticalcapabilities of a hyper-crystal based on the TiN/(Al,Sc)N HMM materialsystem, simulations were carried out on both nanotubes as well asnanospheres (FIGS. 12(A) and 12(B)). The data shows broadbandabsorption >93% (for the nanotubes) and >89% (for the nanospheres)across the visible spectrum while significantly suppressing absorptionin the NIR. For the nanosphere geometry, these absorption values occurfor a monolayer thickness of only 540 nm thick monolayer. Furthermore,the nanosphere geometry could show even less angle-dependence comparedto the transferrable nanotube structures given its high symmetry. TheTiN-based visible absorbers will be fully characterized (via experimentsand simulations) using similar strategies to those outlined for theAZO/ZnO systems.

In some embodiments, a device having metamaterial nanostructures mayinclude a substrate; and metamaterial nanostructures engaged to thesubstrate to form an optical layer to interact with light incident tothe optical layer to exhibit optical absorption or transmission that issubstantially uniform over a spectral range of different opticalwavelengths associated with materials and structural features of themetamaterial nanostructures, each metamaterial nanostructure includingdifferent material layers that are interleaved to form a multi-layernanostructure. The different material layers that are interleaved in ametamaterial nanostructure are concentric layers around an axis to forma cylinder and wherein the axis of the cylinder is perpendicular to alocal contact area with the substrate. The substrate is a flexiblesubstrate to permit bending of the optical layer. The different materiallayers include semiconductor and dielectric materials and the substrateis a polymer material. The different material layers that areinterleaved in a metamaterial nanostructure are concentric layers arounda center to form a sphere. The different material layers that areinterleaved in a metamaterial nanostructure. The different materiallayers include semiconductor and dielectric layers.

In some embodiments, a device having metamaterial nanostructures,comprising a substrate that is flexible; and an array of metamaterialnanopillars engaged to the substrate to form an optical layer, eachmetamaterial nanopillar including alternating semiconductor anddielectric layers to exhibit a plasmonic response to incident light toexhibit optical absorption or transmission that is substantially uniformover a spectral range of different optical wavelengths associated withmaterials and structural features of the metamaterial nanopillars. Insome designs, each nanopillar can be a solid cylinder. In other designs,each nanopillar can be a hollow cylinder with a center void as a tube.The different material layers include semiconductor and dielectricmaterials and the substrate is a polymer material.

In some embodiments, a method for making a device having metamaterialnanostructures, comprising forming metamaterial nanostructures on afirst substrate by depositing and patterning the deposited materials onthe first substrate to form distinctive structures on the firstsubstrate as the metamaterial nanostructures, wherein materials andstructural features of the metamaterial nanostructures are designed toexhibit optical absorption or transmission that is substantially uniformover a spectral range of different optical wavelengths, separating themetamaterial nanostructures from the first substrate, and transferringthe metamaterial nanostructures onto a second substrate that isdifferent from the first substrate. The first substrate is asemiconductor substrate and the second substrate is a flexible polymersubstrate. Each metamaterial nanostructure includes different materiallayers that are interleaved to form a multi-layer nanostructure, andwherein the method further includes performing an atomic layerdeposition (ALD) process to sequentially form the different materiallayers that are interleaved. The method may also include selectingmaterials in each metamaterial nanostructure to render the spectralrange of different optical wavelengths to be in an infrared range. Themethod may also include selecting materials and one or more structuralfeatures in each metamaterial nanostructure to set the spectral range ofdifferent optical wavelengths. The method may also include selecting, inaddition to the selection of the materials and the one or morestructural features in each metamaterial nanostructure, the relativespacing between different metamaterial nanostructures on the secondsubstrate to set the spectral range of different optical wavelengths.The different material layers that are interleaved and are concentriclayers around an axis to form a cylinder and wherein the axis of thecylinder is perpendicular to a local contact area with the firstsubstrate. The different material layers that are interleaved and areconcentric layers around a center to form a sphere.

In some embodiments, a structure with multilayered metamaterialsincludes alternating layers of semiconductor materials, and a secondarysubstrate to which the alternating layers of dielectric and plasmonicmaterials formed on a primary substrate is transferred. The plasmonicmaterial has a carrier concentration that exhibits plasmons in a targetoperating frequency band. The alternating layers of dielectric andplasmonic materials may have a sphere shape in which a number ofconcentric dielectric and plasmonic layers alternate with each other.Alternatively, the alternating layers of dielectric and plasmonicmaterials may have a cylinder shape in which a number of coaxialdielectric and plasmonic layers alternate with each other. Here, theplasmonic material may include Transparent Conduction Oxide (TCO), DopedOxide layers, Aluminum Zinc Oxide (AZO), Indium Tin Oxide (ITO), orTitanium Nitride (TiN), or at least one of the above materials. Thedielectric material may include Zinc Oxide (ZnO), Aluminium Nitride(AlN), or Scandium Nitride (ScN), or at least one of the abovematerials. The secondary substrate may include a polydimethylsiloxane(PDMS) substrate.

In some embodiments, a metamaterial structure may include at least oneof unit cylindrical structure. The unit cylindrical structure mayinclude a core formed along an axial portion of the unit cylindricalstructure, one or more first layers including a semiconductor material,and one or more second layers including a semiconductor material dopedwith a metal impurity. The first and second layers have concentriccylinder shapes and alternate along a radial direction of thecylindrical structure. The second layer has more free electrons than thefirst layer and has less free electrons than those the metal itself has.The unit cylindrical structures form an array in which two or more unitcylindrical structures are periodically arranged on a substrate. The twoor more unit cylindrical structures are periodically arranged such thatcores of four neighboring unit cylindrical structures form a diamondshape.

In some embodiments, a metamaterial structure may include at least oneof unit spherical structures. The unit spherical structure may include acore formed at a center of the spherical cylindrical structure, one ormore first layers including a semiconductor material, and one or moresecond layers including a semiconductor material doped with a metalimpurity. The first and second layers have concentric sphere shapes andalternate along a radial direction of the spherical structure. Here, thesecond layer has more free electrons than the first layer and has lessfree electrons than those the metal itself has. The unit cylindricalstructures form an array in which two or more unit cylindricalstructures are periodically arranged on a substrate. The first layer caninclude Zinc Oxide (ZnO), Aluminium Nitride (AlN), or Scandium Nitride(ScN). The second layer can include Transparent Conduction Oxide (TCO),Doped Oxide layers, Aluminum Zinc Oxide (AZO), Indium Tin Oxide (ITO),or Titanium Nitride (TiN).

The THMMP implemented based on the disclosed technology shows tunable,selective, and broadband near-perfect, omnidirectional absorption. Thebroad absorption bandwidth is a result of the coalescence of absorptionpeaks due to bulk or surface plasmon-polaritons in the AZO and lossymodes that exist due to the hyperbolic dispersion of the arrays.

With the ability to remove these particles from their growth substrates,the arrays may be transferred to visibly transparent and mechanicallyflexible substrates while maintaining their broadband absorption in theNIR. The synthetic strategies presented here are universal and can beapplied to other nanoparticle systems. Not only would this enable a hostof different materials that operate at distinct parts of theelectromagnetic spectrum, but it is anticipated that novel light-matterinteractions that have yet to be explored would be uncovered.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document should not be understoodas requiring such separation in all embodiments.

Additional enhancements and variations can be made based on what isdescribed and illustrated in this patent document.

1-15. (canceled)
 16. A method for making a device having metamaterialnanostructures, comprising: forming metamaterial nanostructures on afirst substrate by depositing and patterning the deposited materials onthe first substrate to form distinctive structures on the firstsubstrate as the metamaterial nanostructures, wherein materials andstructural features of the metamaterial nanostructures are designed toexhibit optical absorption or transmission that is substantially uniformover a spectral range of different optical wavelengths; separating themetamaterial nanostructures from the first substrate; and transferringthe metamaterial nanostructures onto a second substrate that isdifferent from the first substrate.
 17. The method as in claim 16,wherein the first substrate is a semiconductor substrate and the secondsubstrate is a flexible polymer substrate.
 18. The method as in claim16, wherein each metamaterial nanostructure includes different materiallayers that are interleaved to form a multi-layer nanostructure, andwherein the method further includes performing an atomic layerdeposition (ALD) process to sequentially form the different materiallayers that are interleaved.
 19. The method as in claim 16, furthercomprising selecting materials in each metamaterial nanostructure torender the spectral range of different optical wavelengths to be in aninfrared range.
 20. The method as in claim 16, further comprisingselecting materials and one or more structural features in eachmetamaterial nanostructure to set the spectral range of differentoptical wavelengths.
 21. The method as in claim 20, further comprisingselecting, in addition to the selection of the materials and the one ormore structural features in each metamaterial nanostructure, therelative spacing between different metamaterial nanostructures on thesecond substrate to set the spectral range of different opticalwavelengths.
 22. The method as in claim 16, wherein the differentmaterial layers that are interleaved and are concentric layers around anaxis to form a cylinder and wherein the axis of the cylinder isperpendicular to a local contact area with the first substrate.
 23. Themethod as in claim 16, wherein the different material layers that areinterleaved and are concentric layers around a center to form a sphere.24. A method of fabricating a structure with nanoparticles, comprising:generating a silicon hole array template using nanosphere lithography;filling holes of the silicon hole array with multilayered structures ofhyperbolic metamaterials (HMMs) using an atomic layer deposition;etching, using a reactive ion etching process, a top layer of aresulting structure of the filling operation, thereby exposingunderlying silicon of the silicon hole array; and removing siliconsurrounding hyperbolic metamaterial to leave behind an array of packedmultilayered structures of HMIs.
 25. The method as in claim 24, whereinthe multilayered structures of HMMs comprise a plurality of sphericalnanoparticles in which a number of concentric dielectric and plasmoniclayers alternate with each other.
 26. The method as in claim 25, whereinthe plasmonic layer includes Transparent Conduction Oxide (TCO), DopedOxide, Aluminum Zinc Oxide (AZO), Indium Tin Oxide (ITO), or TitaniumNitride (TiN).
 27. The method as in claim 25, wherein the dielectriclayer includes Zinc Oxide (ZnO), Aluminium Nitride (AlN), or ScandiumNitride (ScN).
 28. The method as in claim 24, wherein the multilayeredstructures of HMIs comprise a plurality of cylindrical nanotubes inwhich a number of coaxial dielectric and plasmonic layers alternate witheach other.
 29. The method as in claim 28, wherein the plasmonic layerincludes Transparent Conduction Oxide (TCO), Doped Oxide, Aluminum ZincOxide (AZO), Indium Tin Oxide (ITO), or Titanium Nitride (TiN).
 30. Themethod as in claim 28, wherein the dielectric layer includes Zinc Oxide(ZnO), Aluminium Nitride (AlN), or Scandium Nitride (ScN).
 31. Themethod as in claim 28, wherein: a length of the cylindrical nanotube isin a range of from about 100 nm to about 50 μm; a total diameter of thecylindrical nanotube is in a range of from about 200 nm to about 5 μm;an air core diameter of cylindrical nanotube is in a range of from about50 nm to about 2 μm; a number of the HMM periods is in a range of fromabout 2 to about 100; a thickness of the alternating plasmonic layer isin a range of from about 5 nm to about 200 nm; a thickness of thealternating dielectric layer is in a range of from about 5 nm to about200 nm; a distance between centers of adjacent HMM structures is in arange of from about 100 nm to about 20 μm; a distance between adjacentHMM structures is in a range of from about 0 to about 10 μm; and adeposition temperature of the cylindrical nanotubes is in a range offrom about 100° C. to about 300° C.
 32. The method as in claim 24,further comprising: spin coating a film of polydimethylsiloxane (PDMS)on top of the layered metamaterial structure arranged on a remainingsilicon substrate; curing an output of the spin coating operation; andetching away the remaining silicon substrate from cured structureresulting from the curing.
 33. The method as in claim 32, wherein theetching is performed using xenon difluoride.
 34. The method as in claim32, further including cleaning the silicon substrate using O₂ plasma.